Even though the alternative technique of searching for mines which seeks to identify the mine is well established, the sweeping of mines, although it is a traditional tactic, maintains its considerable importance.
The sweeping consists in moving in the vicinity of the mine with devices which emulate the effect of the passage of a ship in order to explode the mine.
Moored mines, especially those of the impact type, have been shown to be easily swept with normal mechanical sweeping systems and the elimination of entire fields of this kind no longer constitutes a major problem.
However, mechanical sweeping is not found to be effective with the more modern influence mines which are positioned directly on the sea bed at depths of less than 100 meters and are manufactured in such a way as to activate by the influence of the magnetic mass a ship, or its noise or the pressure variation caused by the passage of a ship and, then, following the activation, explode.
In other words, these mines are characterised by the presence of sensors which are capable of detecting the signature of surface or underwater naval vessels and they therefore await explosion when this signature corresponds to a predetermined target.
Amongst the types of signature there are, as mentioned, the magnetic type, the acoustic type and the pressure due to the movement of water connected to the movement of a ship.
The optimal limit of use of influence mines from the sea bed is with a maximum sea bed of around 50-60 meters .
Influence sweeping therefore causes the explosion of a mine using for this purpose precisely the principle of triggering the mine.
Magnetic influence sweeping and acoustic influence sweeping are the most widespread and they comprise devices which are able to generate, respectively, suitable magnetic fields using coils or permanent magnets and acoustic noise using mechanical or electro-acoustic devices.
However, the main difficulties are found in the influence sweeping of pressure mines and in effect, at present, there are no known solutions actually used in practice.
Solutions have also been proposed in the past which are able to reproduce in the proximity of the surface the movement of water and the consequent underlying reduction in pressure of a ship by the pulling of shapes with overall dimensions comparable to that of the ships which are presumably the target of the mine (patent document U.S. Pat. No. 2,967,504).
A second solution prior art, illustrated in patent document U.S. Pat. No. 5,701,839, teaches the generation of a movement of air, a sort of suction, directed from the surface towards the underlying water, which is also able to simulate the negative pressure caused by the passage of a ship.
Both the above-mentioned prior art solutions require large-sized apparatuses and they have been found to be difficult to implement in practice, also in terms of costs and difficulty of use.
The provision is also known, from patent document DE 40 10 686, of a plurality of hydraulic suction machines supported by a floating body.
The prior patent document U.S. Pat. No. 3,012,534 teaches the alteration of the pressure field, designed to activate pressure mines. The above-mentioned alteration of the pressure field is achieved by means of a large tube, kept immersed in a horizontal position, constrained to surface floats and having inside it one or more hydraulic machines which are able to pump water from the inside of the tube towards the outside. This forced circulation of water alters the pressure field.
Both these latter solutions, as they are constrained to the water surface, have not been found to be fully effective in the presence of direct mines for example to strike underwater targets or moving on deeper sea beds.
More specifically, a drawback connected to the use of the latter solution is the impossibility of quickly varying the immersion level, which is often due to changeable operating conditions, often in a rapid fashion.